In various business and technical environments, color is essential as a component of communication, in part because color facilitates the sharing of knowledge and ideas. Color images are commonly represented as one or more separations, each separation comprising a set of color density signals for a single primary or secondary color. Color density signals are commonly represented as image pixels which, in a digital image embodiment, vary in magnitude from a minimum to a maximum, with a number of gradients corresponding to the bit density of the system. Thus, for example, a common 8-bit system can provide 256 shades for each primary color.
A color, in the digital realm, can be represented as the combination of magnitudes of each color separation, which when viewed together present the combination color. Usually, printer signals include three subtractive primary color signals (i.e., Cyan (C), Magenta (M) and Yellow (Y)) and a Black (K) signal, which together can be considered the printer colorant signals. Each color signal forms a separation and is often processed on its own channel in a color printing system, and when combined together with the other separations forms the color image.
It is desirable to specify document properties in a device-independent fashion in order to facilitate the exchange and reuse of documents where possible. Colors are, therefore, preferably specified in a device-independent color space based on the characteristics of human vision. In order to print or display a given color, it is often necessary to determine the device control values corresponding to specified device-independent color values, because the native control spaces of output devices (e.g., a printer's CMYK values) do not constitute device-independent color spaces. As described for example in US Patent Publication 20040257595 by G. Sharma et al., for a “Two-Dimensional Calibration Architectures for Color Devices,” hereby incorporated by reference in its entirety, the calibration process by which colors are represented in device-independent fashion can normally be accomplished utilizing a three-step procedure.
First, a set of color patches with pre-defined device control values is output on the device and the color of each patch is measured in device-independent color coordinates. Second, utilizing the device control values and the corresponding measured device-independent color values, a “forward device-response function” can be estimated. Third, the “forward device-response function” can be “inverted” to obtain a “device-correction-function”.
The “forward device-response function” of step two represents the mapping from device control values to the device independent color values produced by the device in response to the control values. The “device-correction-function” of step three maps each device-independent color to the device control values that produce the specified device-independent color value on the output device. The “device-correction-function” is typically pre-computed and stored in memory. In order to produce a given color on the output device, the corresponding device-independent color values are mapped through the “device correction-function” to obtain control values. When the device is driven with such control values, a desired color can be produced by an output device in response to device-independent input signals.
As described by G. Sharma et al, a common practice is to separate the “device correction-function” into two parts: a “calibration” function that immediately precedes the device and a “characterization” function, which addresses the device “through” the calibration function. This separation is illustrated in FIG. 1 for the case of a conventional CMYK printer. In FIG. 1, a conventional system 100 is depicted, which can be implemented, for example, as a color (CMYK) printer. The system 100 can be subdivided into a “device-correction function” 105 and a “calibration device” portion 107. The “device correction function” 105 can be further partitioned into characterization and calibration portions, respectively represented by a characterization routine 102 and a calibration unit 104.
A device independent color signal(s) can be provided as input 110 to the characterization routine 102, the output of which can be fed to a calibration unit 104. The output from calibration unit 104 is, in turn, provided to an output device 106 such as a color xerographic printing engine as indicated by an output line 114. Additionally, line 112 indicates alternate CMYK (i.e., fast emulation), where data can be output from a reprint path unit 108 and fed to the calibration unit 104. In FIG. 1, the “calibration device” portion 107 of system 100 can be formed generally from calibration unit 104 and output device 106.
Another example of a calibration system includes U.S. Pat. No. 5,305,119 to Rolleston et al, “Color Printer Calibration Architecture,” which issued on Apr. 19, 1994 and is assigned to Xerox Corporation. U.S. Pat. No. 5,305,119 is generally directed toward a method of calibrating a response of a printer to an image described in terms of calorimetric values. A further example of a calibration method and system is described in U.S. Pat. No. 5,528,386 to Rolleston et al, “Color Printer Calibration Architecture,” which issued on Jun. 18, 1996 and is also assigned to Xerox Corporation. U.S. Pat. No. 5,528,386 generally describes a conventional one-dimensional architecture. U.S. Pat. Nos. 5,305,119 and 5,528,386 are both hereby incorporated by reference in their entirety. An example of multidimensional calibration is also found, for example, in US Patent Publications 20060061783 for “Calibration of Color Devices” by M. Yao, and 20060061782 for “Calibration of Color Devices” by M. Yao, both of which are hereby incorporated by reference in their entirety.
As briefly mentioned above, color management is commonly partitioned into a characterization and a calibration transform for output devices. One goal of calibration is to determine a transform from CMYK to C′M′Y′K′ that maintains a desired printer response in selected regions of color space. Additionally, in commercial products, the calibration transform must balance computational efficiency with a reasonable memory size so that it can be incorporated within high-speed real-time printing architectures. Hence, calibration architectures vary in the degree of control provided provide and the underlying cost (i.e. required measurements), storage and/or computation.
The purpose of a “calibration transformation” described above is to facilitate a trade-off. Unlike the “full device-correction function,” calibration transformation provides control of the output device in a limited and exclusive fashion. In comparison to the full device-correction function, however, the calibration transformation also offers significant advantages in that it requires substantially reduced measurement effort and a substantially lower computational effort. The lower computational effort requirement allows such a process, and associated components, to be incorporated in high-speed, real-time printing systems and image-processing chains for which the full device-correction function may be too computationally and/or memory intensive. For color output devices, particularly those utilized in the printing arts, calibration can be performed for the Black (K) channel independently and for the Cyan (C), Magenta (M), and Yellow (Y) channels either independently or together.
Moreover, as will be described in more detail below, it is also desirable to control gray-component replacement in such systems. Standard printer calibration performed with 1-D tone reproduction curves (TRCs) applied channelwise to each C, M, Y and K channel, does not take colorant interactions into account. Specifically, for many devices the interaction between the black (K) channel and C, M and/or Y colorants, as defined by a gray component replacement (GCR) strategy, is crucial to the accuracy and consistency of the rendered color. Recently, printer calibration techniques have been proposed that account for interactions among the Cyan, Magenta and Yellow colorants by means of 2-D or multi-axis color transforms (e.g., as described by G. Sharma et al. in US Application 20040257595). However, these methods still ignore the significant interaction between K and C,M,Y. A calibration technique that exploits the knowledge of this interaction would, therefore, be desirable.
As an example, traditional one-dimensional (1-D) calibration is implemented using simple 1-D look-up tables (LUTs) to transform from CMYK input to a C′M′Y′K′ output. The one-dimensional representation is the most cost effective, but significantly limits the control available over the device color gamut. On the other hand, a three-dimensional (3-D) calibration (3 to 1 LUTs for CMY, 1-D for K) and four-dimensional (4-D) calibration transforms enable significantly more control but tend to require prohibitively large measurements, storage and/or real-time computation. As an intermediate alternative, 2-D and multi-axis calibration transforms have been developed recently that provide superior cost-quality trade-offs (e.g., US Application 20040257595 by G. Sharma et al.). US Application 20040257595 describes a traditional three-color, one-dimensional calibration transformation system, where C, M, and Y inputs to transformations respectively produce C′, M′ and Y′ outputs, where in general the following equations (1A-C) can be employed:C′=f1(C); M′=f2(M); and Y′=f3(Y)  Eq. 1A-1C
It will be appreciated that the dimensionality of the calibration transform dictates a cost versus quality trade-off. To offer an intermediate solution along this trade-off, the 2-D calibration transform was proposed in US Application 20040257595 by Sharma et al., the use of 2-D look-up tables for the control of C, M and Y channels was disclosed with K handled independently via a traditional 1-D look-up table. Mathematically, the use of 2-D look-up tables for the calibration transform for mapping input CMY to output C′M′Y′ may be expressed using two intermediate variables for each output variable that are a function of input CMY. The output C′, M′, and Y′ is then determined by the corresponding two intermediate variables as follows:(s1, t1)=vi1(C, M, Y),   Eq. 2A(s2, t2)=vi2(C, M, Y),   Eq. 2B(s3, t3)=Vi3(C, M, Y); and   Eq. 2CC′=f1(s1, t1),   Eq. 3AM′=f2(s2, t2),   Eq. 3BY′=f3(s3, t3).   Eq. 3C
where sk, tk are intermediate variables that depend on the input CMY. The output C′ is determined by s1 and t1, the output M′ is determined by s2 and t2, and the output Y′ is determined by s3 and t3.
A common shortcoming of the aforementioned calibration transforms (with the exception of a full 4-D transform) is that they all treat the black (K) channel independently and do not account for or address its interactions with the cyan, magenta and/or yellow colorants. Gray-component replacement (GCR) strategies, that determine the mapping from CMY to CMYK, are central to this interaction. The various embodiments disclosed herein address exactly this interaction and develop a novel two-dimensional calibration transform for the K channel.
Disclosed herein is a novel calibration scheme for the K channel that takes into account interactions with C, M and Y. In particular, a two-dimensional calibration look-up table has been derived for the K channel, where a first dimension is input K, and a second dimension is a function of C, M, Y. This multidimensional (2-D) look-up table further exploits the knowledge of the underlying GCR strategy to calibrate along a neutral CMYK locus. In addition, other important axes such as pure K and C=M=Y can also be independently calibrated. The proposed look-up table can be combined with traditional 1-D or 2-D look-up tables for C, M and Y.
Experimental results establish that using a gray-component replacement method enables more accurate rendition of dark colors and also significantly enhances temporal stability of the device. The disclosed system and method is readily implemented via the use of a 2-D look-up table for the K channel, and explicit specification of the GCR strategy in the calibration step. The method could be implemented in the digital front end or DFE (e.g. Xerox® DocuSP™), or directly in the image output terminal/marking engine (IOT). The method is also pertinent to on-line feed-back controls.
Disclosed in embodiments herein is a method for controlling gray component replacement during color calibration of an output device, comprising: receiving image input values for a black channel and at least one non-black channel; applying a pre-defined transformation on said at least one non-black channel to obtain at least one intermediate variable; and determining at least one output black (K) value from a black channel input value and the at least one intermediate variable, to provide control of gray component replacement in the output device.
Further disclosed in embodiments herein is system for controlling gray component replacement during color calibration of a color output device, comprising: a black channel; a plurality of non-black channels; a pre-defined transformation, said transformation applied to said non-black channels to obtain at least one intermediate variable; and at least one output black (K) value determining stage, determining the output black (K) value from a black channel input value and the at least one intermediate variable, to provide control of gray component replacement in the color output device.
Also disclosed in embodiments set forth herein is a digital front end processing system for controlling gray component replacement during color calibration of a color xerographic printer, comprising: a black image data processing channel; cyan, magenta and yellow data processing channels; a pre-defined transformation, said transformation applied to said cyan, magenta and yellow channels to obtain at least one intermediate variable; and at least one output black (K) value determining stage, determining the output black (K) value from a black channel input value and the at least one intermediate variable, to provide control of gray component replacement in the color output device.
The various embodiments described herein are not intended to limit the invention to those embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.